1. Field of Invention
This invention relates generally to fuel control systems for gas turbine engines and, more particularly, to an improvement in such fuel control systems whereby compressor instability is avoided under particular engine operating conditions.
2. Description of the Prior Art
Ideally, a turboshaft gas turbine engine fuel control system schedules fuel flow to the engine's combustor at rates compatible with the power demanded from the engine and with avoidance of compressor instability conditions known as surge or stall. Typical fuel controls achieve this goal for steady state engine operation and for transient cycles of acceleration and deceleration where the cycles are sufficiently separated in time to allow the internal components of the engine to achieve temperature stability. Where time separation between cycles is insufficient for temperature stabilization, the normal fuel schedule might encroach on a region of compressor instability temporarily shifted by the unstabilized temperature conditions within the engine. To avoid such encroachment, expensive and complex systems have been proposed to bleed air from the compressor before the onset of compressor instability. Alternatively, a fuel control system has been proposed which incorporates a time delay mechanism operative to maintain a leaner than normal fuel/air ratio for a preselected duration after the engine reaches idle speed regardless of subsequent power demand. A gas turbine engine fuel control system according to this invention represents an improvement over these and other known systems.